The cobamide-dependent ribonucleotide reductase of Lactobacillum leichmannii has been purified to the point of apparent homogeneity. The purified enzyme catalyzes a transfer of hydrogen from a thiol reductant such as dihydrolipoate to the 2'-deoxyribosyl carbon of dCTP, cobamide coenzyme serving as an intermediate acceptor-donor of hydrogen. The enzyme is allosterically regulated by the various deoxyribonucleoside triphosphates, substrate specificity being wholly dependent on the allosteric effector present. Recent work has shown that despite the complexity of its regulatory behavior, the enzyme lacks subunits, being a single polypeptide of molecular weight 80,000. However, the enzyme does exist in interconvertible oxidized and reduced forms. Current experiments seek to study (1) the regulatory significance of the alternative forms of ribonucleotide reductase; (2) validity of the "methylfolate trap" theory on vitamin B12 action; (3) levels of "methyltransferase" and other enzymes catalyzing interconversions of folate compounds in normal and leukemic blood cells; and (4) studies of the synthesis and degradation of pteroylpolyglutamates in animal cells.